1. Field of the Invention
This invention relates to a press section of a papermaking machine and to a pressure shoe for use in a press section having an extended nip.
2. History of the Prior Art
The concept of a stationary shoe exerting pressure on a rotating drum through a moving paper web transport system produced questions of friction, temperature, tension, and materials. These questions became evident when the transport systems developed a performance inhibiting bulge at the nip. In earlier patents entitled, "Extended Nip Press with Special Belt Reinforcement," U.S. Pat. No. 4,229,253, issued to the Applicant on Oct. 21, 1980 and "Extended Nip Press with Bias Ply Reinforced Belt," U.S. Pat. No. 4,229,254, issued to Michael L. Gill on Oct. 21, 1980, transport belt designs were proposed as answers to some of these questions. A reinforced belt was found to bulge less at the extended nip. As a result, the belt tension, machine part wear, and energy consumption could be reduced. Nevertheless, further reduction in power consumption, frictional forces, and pressure concentrations at the nips of the papermaking machine were still needed.
D. D. Fuller, in his text entitled, Theory and Practice of Lubrication for Engineers, published in 1956, studied the friction and pressure buildups on the surface of variously designed hydrodynamic bearings. His studies indicated the design of the inlet geometry for hydrodynamic bearings had little effect on the frictional forces or pressure buildups at the bearing surface. As a result, prior art in the area of extended nip applications in papermaking machinery indicated little need for specialized nip shoe design.
When Fuller's conclusions were tested, it was unexpectedly discovered that nip shoe design is significantly relevant when compliant or compressible materials are subjected to the hydrodynamic bearings. It was found that the compliant transport systems used in paper making operations exhibit properties which are appreciably different from the noncompliant surfaces tested by Fuller.
Fuller discussed the friction, pressure, and lubrication considerations associated with shafts, metal sliding surfaces on production machine tools, and the interfaces of other metallic components. Such applications required no special hydrodynamic bearing design to maintain an adequate film of lubrication along the interface of contacting metal parts. However, the bearing design was found to have a substantial impact when used with the compliant felts and transport belts common in papermaking machinery.
Data indicated that the compliant transport systems, used to move a paper web through a papermaking machine, "bunched up" at inrunning nips and caused excessive friction, pressure, and power consumption throughout the papermaking machine. A film of lubricant at the interface of a nip shoe and compliant transport system was consistently wiped away by the friction and pressure concentrations at the inrunning nip.
Faced with this dilemma, the extended nip shoe design was modified and eventually a shoe which significantly reduced friction and pressure at the inrunning nip was developed. The novel extended nip shoe design also maintained a film of lubricant at the interface of the compliant transport system and the extended nip shoe. It was concluded that by extending the nip shoe beyond the point where the compliant transport system initially compacts against the shoe and opposing surface, lubricant could be introduced into, and maintained throughout, the shoe-compliant transport system interface.
The disclosed extended nip shoe design decreases the pressures at the inrunning and outrunning nips. A lubricating film at the shoe-compliant transport system interface decreases the frictional forces along that interface. Since the impediments of friction and pressure concentration are decreased, the power required to move the compliant transport system across the extended nip shoe is also reduced. By-products of the decreased friction, pressure, and power consumption include lower operating costs and extended bearing and compliant transport system lives since less tension is required to move the transport system over the shoe. The invention permits increased control of paper web processing time under selected pressures. The extendability of the nip allows lower pressure application to a web of paper over longer time periods. The web processing operation is extended from the previous line of contact between two press rolls to the longer contact time available with the extended nip. This feature may produce a higher quality of processed paper than previously realized under short time but high pressure paper processing.